Thermal imaging & vision systems
Figure 5. An illumination profile example.
demonstrates an example of a batwing illumination profile. If one wishes to achieve constant irradiance on the pixel array of the imager from a flat target, one should also consider a cos3 ( ) drop-off factor in irradiance (E) between the target center and the target edge [W/m2], which is defined as:
Optical efficiency—enclosed energy within the imaging lens FOV This specification defines how much energy will be received by the imaging module and is specified by:
to the diffuser, or by choosing a light source with a smaller divergence angle.
IMAGING MODULE
where E is irradiance, dA is the surface area illuminated by optical power d , R( ) is the distance between the light source to dA defined in Figure 4, and dΩ = dAcos( )/R( )2.
Width of the profile
The width of the profile determines the FOI of the illumination profile. It can be defined as full width half max or 1/e2 of the maximum intensity. To accommodate misalignment between the imaging lens to the imager and the tolerance of the diffuser, FOI is normally designed to be slightly larger than the FOV of the lens to avoid dark pixels. The width of the profile is the convolution of the intensity profile of the light source to the diffuser response to a collimated beam. The wider the input divergence angle to the diffuser, the wider the width and slower the transition slope. A wider and slower transition slope results in more energy falling outside the FOI, which causes optical power loss. The acceptance criteria for such loss can be specified using the following two requirements.
In general, the optical efficiency can be improved by having a collimator lens between the light source and the diffuser to reduce the input angle
Figure 5c illustrates the concept of 2D integration of the illumination profile within FOV.
Optical power drop-off outside FOI
The imaging module consists of an imaging lens assembly, band-pass filter (BPF), and microlens array on the imager. The thickness and material of the backside optical stacks on the imager should be optimised for low back-reflection. Figure 6 shows an illustration of the imaging module.
TOF IMAGING LENS DESIGN CONSIDERATIONS
Since the ToF camera collects light generated by active illumination, the efficiency and uniformity of the light collection on the pixel array greatly affect the overall performance. The lens needs to have a strong collecting power, high transmission, and low stray light. The following are design considerations for ToF lenses, which are distinct from traditional RGB camera lenses.
Figure 6. Illustration of the imaging module. Figure 4. Irradiance distribution vs. intensity. 48 April 2025 Instrumentation Monthly
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